This invention relates to an arrangement for the thermal insulation of high-pressure vessels, pipes, or similar structures, and includes a vacuum-tight sheath with a thin wall and a vacuum-tight sealable opening for filling with insulating material. The invention furthermore relates to a process for the fabrication of an arrangement of this type.
Insulation systems utilizing a vacuum have been known for a long time from cryogenic technology, for example, in the form of so-called DEWAR flasks. These systems are based upon avoiding heat transfer by thermal convection. Because heat transfer by conduction is practically eliminated at the same time, the heat can only be transferred by radiation. In order to counter this, the surfaces of these systems, insofar as heat transfer by radiation could occur between them, are generally made reflective, or the radiation is reduced by the placement of low-emission foils.
From U.S. Pat. No. 1,071,817 a pressure-resistant vessel is known, which also has a pressure-resistant outer casing. The space between the casing and the vessel is evacuated in order to achieve the desired thermal insulation. In addition, the space between casing and vessel is filled with a finely divided material in powder form with absorption properties in order to absorb gas molecules released from the casing or entering through leaks to preserve the vacuum for as long as possible. Either sublimed SiO.sub.2 or carbon in powder form is used as the filling material.
Furthermore, from U.S. Pat. No. 2,000,882 as well as from U.S. Pat. No. 2,863,584 double-walled vessels, whose walls are held apart from one another by spacers, are known. The space between the walls is filled with a porous filling material, e.g., diatomaceous earth. A piping system within the filling space is used in order to evacuate the filling space. The atmospheric pressure acting against the walls is intercepted by the spacers. These spacers and the evacuating pipes represent thermal bridges, which impair the insulating properties and make use at high temperatures impossible.
According to U.S. Pat. Nos. 2,108,212, the filling of the double-walled vessel with charcoal in powdered form according to U.S. Pat. No. 1,071,817 has not proved satisfactory because this material is particularly hygroscopic, and as a result the otherwise good insulating properties are counteracted, insofar as there is no heating simultaneously with filling in order to drive off the moisture.
Moreover, according to U.S. Pat. No. 2,108,212 the briquetting of the carbon or pressing into plates and the positioning of the material in this form between the two walls of an evacuated sheath or of an evacuable double-walled vessel to provide support from one wall to the other was attempted in order to achieve improved strength. It was found, however, that the mechanical compression of the finely powdered carbon leads to the caking of this material and that as a result the otherwise good insulating properties are lost. This effect also occurs when carbon in powder form is filled between flexible thin walls with no spacers and is expected to serve as the spacing material. The disadvantages of spacers are then indeed avoided, moreover the use of briquetted filling plates of carbon powder--with relatively poor thermal insulation is eliminated. The filling of carbon powder between the flexible walls is, however, also strongly compressed by atmospheric pressure after evacuation with the result that the individual carbon particles cake together and produce an undesirable high solid thermal conductivity.
In order to avoid the disadvantages occurring from the use of carbon powder for self-supporting insulating elements, it was proposed in U.S. Pat. No. 2,108,212 that the carbon powder be pressed into a granulate before filling because then the solid thermal conductivity within the carbon is very small as a result of the minimum contact of the individual granules, and the worrisome caking of the carbon particles is certainly avoided. At the same time a relatively large surface area remains, and thereby good absorption properties of the carbon material are retained. Moreover, the granulate should maintain the space between the sheath walls certainly and reliably. A disadvantage is that in the case of self-supporting plates filled with carbon granulate the additional granulate production which is necessary results in higher costs and particularly the use of plates of this type or insulating elements is not possible under high-pressure conditions because then the individual granules break and cake under the working of the outer pressure.
From U.S. Pat. No. 2,164,143 a process for the fabrication of thermal insulating elements, which are designed to be self-supporting (i.e., without spacers) and which consist of a flexible, thin sheath that is filled with a porous material in powder form, e.g., carbon or diatomaceous earth and then evacuated, is known. According to this known process, a sheath of thin, flexible material is filled with the porous material in powder form. Thereby the sheath is enclosed in a supporting form, and the filling material is pressed together so that the filling material exerts a mechanical pressure against the sheath. Finally, the sheath is closed, and gas is evacuated from the sheath, whereby the mechanical pressure of the filling material is exerted against the outer atmospheric pressure and opposes it. This known fabrication process satisfies its purpose, namely, the fabrication of dimensionally exact insulating elements through corresponding pre-compression of the insulating material in filling, only for insulating elements used with an outer atmospheric pressure because only then is the compensation between the pre-compression pressure working from the inside against the sheath and the outside atmospheric pressure guaranteed. Moreover, the elements fabricated by this process have an evacuating pipe system within the filling space, which makes the elements inappropriate for applications in high-temperature regions because of the resulting thermal bridges.
For high-pressure and high-temperature applications an evacuated insulating element, which also possesses a sheath of thin, flexible foil material or sheet as well as an insulating material within the sheath or spacer, was proposed in DE-OS 26 15 299. Glass-fiber felt is proposed as the insulating material. As a result of its relatively great hardness, the fibers of the glass-fiber felt generally touch one another by point contact even after the evacuation so that at the crossing points of two fibers, respectively, a high thermal resistance results; and the thermal conductivity through the element possesses the desired low value. The thermal conductivity .lambda. is approximately a factor of 10 smaller for this element than for known hard-foam plates or plates of mineral or glass fibers.
The known evacuated element from DE-OS 26 15 299 possesses the disadvantage that only with the utilization of absorption substance within the filling space can the vacuum be maintained under a permissible final pressure of approximately 10.sup.-1 torr because the glass-fiber felt possesses no absorption properties. Even with the use of absorber substances only lifetimes of less than 30 years can be achieved. Moreover, the glass fibers cake at over 400.degree. C. and then have unfavorable high body thermal conductivity. As a result of the very low packing density of the glass fibers, which occupy approximately 10% of the volume, and of the disordered orientation, the glass fiber felt is moreover only reversibly compressible and returns to the unloaded condition after pressure release. Thence, the elements under alternating pressure loading experience unpredictable microscopic deformations which can lead to the damaging of the glass-fiber felt and the sheath.